1. Field of the Invention
The present invention relates to fuel cell electric power systems, and particularly to methods and apparatus for improving the performance of fuel cell stacks in fuel cell electric power systems.
2. Description of the Related Art
Fuel cells electrochemically react a fuel stream comprising hydrogen and an oxidant stream comprising oxygen to generate an electric current. Fuel cell electric power systems employing stacks of fuel cells are used in a variety of transportation, portable and stationary power applications.
Performance losses have been observed in fuel cells employing catalysts comprising platinum (Pt.). For example, under constant current conditions the voltage of polymer electrolyte membrane fuel cells typically is initially as high as 0.9 V, but begins to fall fairly quickly. Similarly, under constant voltage conditions, particularly at higher voltages, the initial current drops from its original output value. This performance loss has been attributed to various sources, including carbon monoxide poisoning of the catalyst and loss of cathode catalyst activity due to adsorption of oxides from water.
Several approaches have been employed for reversing this performance degradation. For example, U.S. Pat. No. 5,601,936 discloses applying a reverse DC potential to the anode and cathode of a fuel cell. CA 2,284,589 discloses pulsing the anode potential by temporarily shorting the anode and cathode or by producing a positive voltage pulse with an external DC voltage source. Conversely, US 2001/0044040 A1 discloses switching the cathode to an output load to reduce the cell voltage at a pulse width effective to reverse performance degradation. U.S. Pat. No. 6,096,448 discloses fuel starving a fuel cell stack by connecting a transient load to the stack. Both U.S. Pat. No. 6,096,449 and U.S. Pat. No. 6,451,470 B1 disclose periodically shorting fuel cells.
U.S. Pat. No. 6,096,449 also discloses a shunt controller coupled to numerous shunt control circuits corresponding to each fuel cell in the system. The shunt controller also comprises a pair of voltage sensors, a bypass circuit coupling the anode and cathode together, and a current sensor associated with each fuel cell. The shunt controller further controls a fuel gas shut-off control valve for terminating supply of fuel gas to a fuel cell.
These approaches have one or more disadvantages. First, they may result in power fluctuations to the external load when the fuel cell stack is shorted or its current or voltage is pulsed. This is problematic in applications where the external load is intolerant of the magnitude and/or frequency of such power fluctuations. Alternatively, the external load may be disconnected from the fuel cell stack during pulsing, undesirably interrupting power supply to the load.
Second, the control systems involved in implementing such approaches tend to be overly complex. The complexity of the shunt controller in U.S. Pat. No. 6,096,449, for example, may undesirably increase the cost and decrease operational reliability of the overall system.
It is desirable to have a method and apparatus for operating a fuel cell electric power system that not only increases the performance of the fuel cell stack, but is also relatively inexpensive, reliable and electrically efficient. The present invention addresses the disadvantages of conventional fuel cell power systems and provides further related advantages.